11.2 Design Based on Allowable Torque for Optimum Life (For HTD TRUE METRIC drives only)

Transcription

1 11.2 Design Based on Allowable Torque for Optimum Life (For HTD TRUE METRIC drives only) If all the values and coefficients which govern the belt life are established experimentally. then by calculating simultaneously belt life and horsepower, a set of values can be derived for which a somewhat reduced horsepower rating will yield substantially longer life. Such evaluations have been made for the TRUE METRIC 3mm and 5mm belts, and the findings have been put into the form of Tables 30 thru 33 which show variable allowable torque values as a function of rpm and pulley diameter as well. These values are given for specific belt widths. The torque values used with these tables must take into account the actual torque requirements derived by experiment or calculation, and must be multiplied by a service factor given in Table 38. The service factor must be chosen based on actual operating conditions. We will call this increased torque the Design Torque. By substituting torque values from Tables 30 thru 33 into [15] the horsepower Tables 34 thru 37 have been compiled. If these tables are used in lieu of Tables 28 and 29 the resulting belt widths will have to be increased, but belt life will also be substantially improved. PLEASE NOTE THAT THIS METHOD OF BELT SELECTION BASED ON OPTIMUM LIFE, WAS DEVELOPED BY UNIROYAL INC. ONLY RECENTLY AND IS APPLICABLE ONLY TO THE HID TRUE METRIC BELTS. THERE ARE NO SIMILAR COMPUTATIONAL MEANS AVAILABLE FOR THE MXL, XL AND L TYPE BELTS THESE HAVE TO BE CHOSEN IN THE CONVENTIONAL WAY BASED ON HORSEPOWER. THE APPLICABLE PROCEDURE IS GIVEN IN THE FURTHER TEXT. If specific belt life values are required for the TRUE METRlC belts, SDP Application Engineering will be glad to provide these if the basic design data is supplied. A minicomputer program exists, and telephone inquiries can be promptly expedited Drive Selection Procedure Based on Horsepower Step 1: Determine design horsepower By definition design horsepower is a multiple of the rated horsepower, which takes into account overload and the specific drive characteristics. Dhp = hp x SF [16] Service Factor SF is a multiplier always greater than one which is established based or service requirements. Upon determining the class of driver per Table 39 the Basic Service Factor per Table 38 is selected. In case of a Speed-up Drive an additional factor is added to SF, whereas per Table 41 unusual conditions will further modify SF. Step 2. Select belt pitch From belt pitch selection graph Fig. 13 select the belt type which will be used. There is usually more than one choice which can be considered. If a belt of stronger design is used it will result in a narrower belt. In many instances the selection will be made based on: total cost of drive, compatibility with previous designs, required flexibility of drive belt, depth of engagement between belt & pulley, availability of particular belt length, and finally personal preference of the designer. 600

2 FOR 1 INCH WIDE BELT BASED ON TENSILE STRENGTH Note: Exam To obtain HP capacity for widths other than one inch use value in table, multiplied by width factor. ple:a one-inch-wide MXL belt running on a 30 MXL driver pulley at 7000 RPM has a capacity of 1.35 HP. To find the HP capacity of a 3/8-inch-wide MXL belt (6Z16-xxx037) multiply 1.35 X HP per 3/8-Inch-wide MXL belt. Area beneath dotted line: This pulley and RPM can be used only if a Corresponding reduction in belt service life is allowable. Area beneath interrupted line: This pulley and RPM can be used only if a simultaneous reduction in belt service life and torque reduction is taken into account. TABLE

3 FOR 1 INCH WIDE BELT BASED ON TENSILE STRENGTH Note: Example: To obtain HP capacity for widths other than one inch use value in table, multiplied by width factor. A one-inch-wide XL belt runningona 30 XL driver pulley at 7000 RPM has a capacity of 3.84 HP. To find the HP capacity of a 3/8-inch-wide XL belt (6Z3-XXXO37) multiply 3.84 X 0.29 = 1.11 HP per 3/8-inch-wide XL belt. Area beneath dotted line: This pulley and RPM can be used only if a corresponding reduction in belt service life is allowable. TABLE

4 FOR 1 INCH WIDE BELT BASED ON TENSILE STRENGTH Note: Example: To obtain HP capacity for widths other than one inch use value in table, multiplied by width factor. A one inch wide L belt running on a 22 L driver pulley at 4000 RPM has a capacity of 4.23 HP. To find the HP capacity of a ½ inch wide L belt (6R4-XXX050) multiply 4.23 X HP per ½ inch wide L belt. Area beneath dotted line: This pulley and RPM can be used only if a corresponding reduction in belt service life is allowable. TABLE

5 FOR 1 INCH WIDE BELT BASED ON TENSILE STRENGTH Important Note: This table is computed based on tensile strength similarly to tables for 0.080, and inch pitch belts. DO NOT USE FOR DESIGN. USE FOR COMPARISON ONLY. Tables later In this chapter are based on allowable torque, and the resulting computation yields optimum belt life. Note: To obtain HP capacity for widths other than one inch use value in table, multiplied by width factor. TABLE

6 FOR 1 INCH WIDE BELT BASED ON TENSILE STRENGTH Important Note: This table is computed based on tensile strength similarly to tables for 0.080, and inch pitch belts. DO NOT USE FOR DESIGN. USE FOR COMPARISON ONLY. Tables later In this chapter are based on allowable torque, and the resulting computation yields optimum belt life. Note: To obtain HP capacity for widths other than one inch use value in table, multiplied by width factor. TABLE

7 606

8 TABLE 30 6mm (0.24in.) Wide Belt 607

9 TABLE 31 9mm (0.35 in.) Wide Belt 608

10 TABLE 32 9mm(0.35 in.) Wide Belt 609

11 TABLE 33 15mm (0.59 in.) Wide Belt 610

12 BASED ON ALLOWABLE TORQUE FOR OPTIMUM LIFE TABLE 34 6mm (0.24 in.) Wide Belt 611

13 BASED ON ALLOWABLE TORQUE FOR OPTIMUM LIFE TABLE 35 9mm (0.35 in.) Wide Belt 612

14 BASED ON ALLOWABLE TORQUE FOR OPTIMUM LIFE TABLE 36 9mm (0.35 in.) Wide Belt 613

15 BELT ON ALLOWABLE TORQUE FOR OPTIMUM LIFE TABLE 37 15mm (0.59 in.) Wide Belt 614

16 615

17 Speed-Up Ratio Range 1 Thru Thru Thru Thru Thru Over Additional Factor None TABLE 40 SPEED-UP DRIVES TABLE 41 UNUSUAL CONDITIONS For 24-hour continuous operation and/or use of an idler, add 0.2 to Service Factor. For intermittent or seasonal operation, deduct 0.2 from Service Factor. 616

18 617

19 618

20 Figure

21 It is worth noting that the 3mm and 5mm TRUE METRIC belts are capable of carrying loads normally associated with much coarser pitch belts. As a result, the use of TRUE METRIC belts will yield the following advantages: smaller overall size, narrower belts, ultimately lower cost. Step 3. Select pulley combination Use Drive Ratio Tables (Table 22) and choose pulleys depending on the space available; TRUE METRIC pulley dimensions are given in Tables 18 and 19. Check surface speed of smaller pulley BS (fpm) = x pd (in) x rpm [17] BS (m/s) = x pd (mm) x rpm [18] which should not exceed 6500 f pm for TRUE METRIC (10,000 for MXL and 5500 for XL). Check compliance with requirement given on Table 4 for minimum pulley diameters. Step 4. Determine belt length and nominal center distance Choice of longer belt length has the effect of increasing belt life. On the other hand it also increases the envelope dimensions of the drive, as well as its costs. It is desirable to choose belt lengths which are available as standard stock items. For calculation of accurate center distances see separate section with formulas and center distance factor-tables (Table 23). Step 5. Select belt width From the appropriate Belt Horsepower Capacity Ratings (Tables 25 thru 27) and belt width factors calculate the width of the belt needed to satisfy the calculated Dhp. Step 6. Obtain actual Service Factor This step is introduced to recheck previous steps Te is calculated from [20] and Tc from [14] or obtained from graph Figure 12. Ta for the appropriate belt width is taken from Table 3. If SF is satisfactory, then design is finalized, if not, increase belt width and correspondingly the value of Ta will also increase. 620